Field of the Invention
The invention relates to an integrated memory having writable memory cells. The invention also relates to a method for preventing aging in a memory cell in an integrated memory.
Ferroelectric memories, such as FRAMs or FeRAMs, are writable integrated memories with memory cells which have at least one ferroelectric capacitor as a storage element. That capacitor has a ferroelectric material as a dielectric. That material can be polarized in different ways, corresponding to the storage of different information. The ferroelectric material retains its polarization even when the memory is disconnected from a supply voltage. Therefore, FRAMs are writable, nonvolatile memories. However, ferroelectric memory cells are associated with aging phenomena which result in it not being readily possible to store new information. Thus, for example, the memory cells tend to retain an already programmed state (a so-called "imprint").
U.S. Pat. No. 5,270,967 relates to preventing aging in ferroelectric memory cells by refreshing their ferroelectric capacitors. Refreshing involves applying voltages to the storage capacitor which are higher than those arising during normal operation. During refreshing, those higher voltages are applied to the storage capacitor with alternating signs. The imprint can be reduced by using the method described. In that case, the memory cells are refreshed whenever a particular number of write access operations to the memory cells have been performed.
In U.S. Pat. No. 5,270,967, before a memory cell is refreshed, its content is temporarily stored in a sense amplifier which is subsequently isolated from the memory cell by a switch. A special circuit unit then produces the alternating voltages required for refreshing. At the end of refreshing, the switch is closed and the information temporarily stored in the sense amplifier can be written to the memory cell again to produce the previous state.
A disadvantage of the procedure described above is that the provision of the additional circuit unit, which produces the higher voltages necessary for refreshing, entails a relatively large space requirement.